As demand for ever-shrinking semiconductor devices continues to increase, so too will the demand for improved semiconductor wafer metrology systems. The fabrication of semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control one or more semiconductor layer processes. For example, metrology processes are used to measure one or more characteristics of a wafer such as dimension (e.g., line width, pitch, feature shape. thickness, etc.) of features formed on the wafer during a process step, wherein the quality of the process step can be determined by measuring the one or more characteristic parameters deemed to be critical. In this scenario, a given semiconductor sample may include a set of metrology targets, with film stacks or two-dimensional and three-dimensional patterned structures surrounded by one or more materials of various geometries and properties.
In a diffraction limited system, the spatial resolution of the given metrology tool is provided by the Rayleigh criterion, namely:
                    d        =                              1.22            ⁢                                                  ⁢            λ                    NA                                    Eq        .                                  ⁢        1            
where λ represents the wavelength of the illumination used by the optical system of the metrology tool and NA is the numerical aperture of the optical system of the metrology tool. In the case of ellipsometry-, polarized and unpolarized reflectometry-, and scatterometry-based metrology measurements, quantitative results (e.g., film thickness, critical dimension (CD), and the detailed feature shape) are generally impacted significantly by diffraction. As such, it would be advantageous to provide a metrology system that aids in mitigating the fidelity loss on the measurements as a result of diffraction effects.